1. Field of the Invention
This invention relates to a structure of a direct conversion flat panel X-ray detector that provides for automatic cancellation of ghost images. More particularly, the invention relates to an X-ray image flat panel detector comprising a direct X-ray to charge converter, a readout thin film transistor array and a transparent substrate, in which the after image or ghosting effect, due to charge trapping within the charge converter, is automatically and instantly suppressed by a provision of a layer of luminophor on the rear of the substrate.
2. Description of the Prior Art
During the last 15 years, many efforts have been directed towards the development of digital detectors for X-ray imaging. These detectors benefit from a much wider dynamic range and improved detection efficiency than film, allowing for much better imaging quality. These emerging detectors can be divided into two groups, one group where the X-rays are converted into light photons which are then detected by a light sensitive device i.e. luminophor screen+photodiodes or charge coupled device (CCD), and the other group where the X-rays are directly converted into electric charges, i.e. direct detection by a semi-conductor such as silicon, germanium, cadmium telluride, thallium bromide or amorphous material like selenium.
The use of selenium as a direct converter, meaning direct generation of electrical charges under X-ray exposure, simplifies the structure of the detector leading to substantial improvement in production yield and cost. It also has performance advantages such as resolution and sensitivity. Nevertheless, the collection of these electrical charges requires the use of substantial electric fields and can be incomplete because of trapping effects leading to image retention and ghosting. As described in U.S. Pat. No. 5,880,472 by Bradley Trent Polischuk and Alain Jean, the selenium direct converter has a multilayer structure consisting of xe2x80x9cpxe2x80x9d, xe2x80x9cixe2x80x9d and xe2x80x9cnxe2x80x9d layers. The xe2x80x9cpxe2x80x9d and xe2x80x9cnxe2x80x9d layers are no more than a few micrometers thick; their role is to block the injection of electrons and holes respectively, allowing for a low dark current under high voltage polarization. The X-ray-to-charge conversion takes place in the thick xe2x80x9cixe2x80x9d layer. The ghosting effect results from charges captured in and released from traps located within the xe2x80x9cixe2x80x9d selenium converter material as well as in the interfaces of the xe2x80x9cpinxe2x80x9d structure. Each trap is associated with a charging and discharging time depending on its energy depth within the amorphous selenium energy gap between a conduction band and a valence band or within trap states located in the xe2x80x9cipxe2x80x9d and xe2x80x9cnixe2x80x9d interfaces. Shallow traps with release time less than a few microseconds are insignificant ( less than 0.5 eV). Deep traps (about  greater than 0.7 eV) which release their charges within a very long time can build up a residual background image which will be detrimental to applications in which switching from intense to low X-ray flux is required, such as in angiography applications.
In indirect converters, most of the after image effect is the result of light emission variations over the emitting surface of the scintillator. It translates into a background non-uniformity which can be instantly compensated for by the addition of an offset in the processing of the image.
In direct converters, such as those using selenium, a significant part of the after image is the result of sensitivity variations under the effect of trapped charges. Such trapped charges whether in the xe2x80x9cinxe2x80x9d interface, xe2x80x9cpixe2x80x9d interface or in the bulk of the xe2x80x9cixe2x80x9d layer, tend to locally modify the electric field profile and thus to change the collection rate. It results in a gain variation and can only be corrected for electronically by tedious and time consuming multiplication operations over the entire array.
In U.S. Pat. No. 5,132,541 by Conrads et al. entitled xe2x80x9cSensor Matrixxe2x80x9d, a direct converter flat panel detector of a similar structure is described. Nevertheless, the charge trapping effect is ignored and no correction method is proposed.
In U.S. Pat. No. 5,396,072 by Schiebel et al. entitled xe2x80x9cX-Ray Image Detectorxe2x80x9d a direct converter flat panel detector is again described showing ways to avoid capacitance coupling with the collecting leads. There is no mention of image ghosting nor of ways to avoid image retention by charge trapping.
In U.S. Pat. No. 5,880,472, already mentioned above, a selenium multilayer structure allowing real time imaging capabilities is disclosed. However, no indication of the level of image retention or of the means by which this effect can be eliminated is given.
In U.S. Pat. No. 6,078,053 by Adam et al., an X-ray image erasure method is disclosed according to which ghost images are erased by simultaneous application of high voltage and light to the X-ray imaging device. However, it provides no automatic cancellation of such ghost images by the actual structure of the device.
Finally, in a paper by Lee D. L., Cheung L. K., and Jeromin L. S., entitled xe2x80x9cA New Digital Detector for Projection Radiographyxe2x80x9d, 1995, SPIE Vol. 2432, pp. 237-249, a direct conversion selenium based thin film transistor (TFT) imaging system is disclosed where a burst of light is triggered to reset the panel after readout, once the high voltage has been switched off. However, this technique, required only to cancel the stored image signal, though it may simultaneously release trapped charges, suffers from the fact that it cannot be used for real time imaging applications, such as in angiography.
There is thus a need for a structure that would provide automatic cancellation of ghost images in direct conversion detectors.
It is an object of the present invention to design a structure of a flat panel direct conversion X-ray detector that provides for automatic cancellation of ghost images.
Another object of the present invention is to apply such structure to selenium flat panel X-ray detectors.
Other objects and advantages of the invention will be apparent from the following description thereof.
The present invention uses the incomplete X-ray absorption and subsequent partial transmission of the converter layer of the flat panel detector, such as an amorphous selenium layer, and of its substrate, to stimulate a luminophor provided on the back of said substrate. It uses as well the light emitted by the luminophor and transmitted through the TFT array built on the glass or other transparent substrate to reach the selenium xe2x80x9cpinxe2x80x9d structure and release the trapped charges. More specifically, as the non-absorbed X-ray beam impinges on the luminophor located on the rear of the transparent substrate, light is generated, which, by transmission through the TFT array, instantly inhibits any trapping of charges, preventing any subsequent alteration of the electric field topography and thus the ghosting effect. Moreover, the trap inhibition process is self controlled. Indeed, if the trapping increases locally as a result of increased X-ray flux, the generated light intensifies and the de-trapping rate increases. To be efficient, the light spectrum emitted by the luminophor should match the absorption spectrum of the traps. Taking into account the energy gap of amorphous selenium, the depth of the traps and the energy level of the interface states of the xe2x80x9cpinxe2x80x9d structure, the wavelength of the luminophor will normally extend from 580 nm to 620 nm. To provide for a minimum transmission through the TFT array, it is desirable that its architecture comprise pixel pads and electrodes made out of transparent conductors such as Indium Tin Oxide (ITO) or that the fill factor of the pixels be partial in order to allow light to flow through the voids. A fill factor of 70% is about the best that can be achieved in current TFT technology process. It allows up to 30% of the light generated by the luminophor to reach the selenium structure, which is sufficient when using a luminophor of average conversion efficiency.
It is already known to use a luminophor for improved recording of X-ray images, as disclosed in U.S. Pat. No. 4,554,453 by Feigt et al. Also, it is known to use a patterned luminophor in a method of X-ray image acquisition by exciting such luminophor with X-ray radiation to emit light by luminescence, as disclosed in U.S. Pat. No. 5,796,113 by Nagli et al. It is, however, unknown and surprising that a luminophor could be made part of a flat panel direct conversion X-ray detector to achieve automatic cancellations of ghost images that would otherwise be produced by such detector.
In greater detail, a preferred embodiment of the present invention provides a direct conversion flat panel X-ray detector with means for automatic cancellation of ghost images, which comprises:
(a) an X-ray to charge converter with a common coplanar electrode;
(b) an array of storage capacitors, having a top electrode which collects the charges delivered by the converter;
(c) a thin film transistor (TFT) matrix wherein each TFT xe2x80x9csourcexe2x80x9d is connected to a corresponding capacitor upper electrode and TFT gate electrode is connected to a line lead common to all TFT gates of a same line as its xe2x80x9cdrainxe2x80x9d which is connected to a readout column lead common to all TFT drains of a same column;
(d) a transparent substrate having a front side and a rear side and on its front side supporting the TFT matrix; and
(e) a layer of luminophor on the rear side of the substrate adapted to emit light under impact of X-ray flux transmitted through the substrate, suitable for automatic cancellation of ghost images that may be trapped in the X-ray to charge converter.
The X-ray to charge converter may be, for example, a thick layer of cadmium telluride or of cadmium selenide, or of thallium bromide or of lead oxide or of amorphous selenium.
The array of storage capacitors and readout TFTs may be encapsulated in a transparent insulator provided with an array of transparent charge collecting pads made of ITO. Also, the array of storage capacitors may have transparent electrodes made of ITO, with the upper electrode being in contact with the charge collecting pads by xe2x80x9cviasxe2x80x9d through the transparent encapsulating material. On the other hand, the array of storage capacitors may have metal electrodes made, for example, of aluminum, titanium, gold, molybdenum or chromium, which electrodes are separated from the scan and readout leads by voids providing light feedthrough.
Also, the TFTs of the array may cover only a small fraction of less than 10% of the pixel area with their source connected to the upper capacitance electrode, their gate connected to the line scan lead and their drain connected to the readout column lead.
The light emitting luminophor covering the rear side of the transparent substrate is preferably a thick X-ray absorbing layer of Zn3(PO4)2; (Zn, Cd) S:Ag; YVO4:Eu; Y2O3:Eu; Y2O2S:Eu or Y2W3O12:Eu. Such luminophor can be combined with an X-ray absorbing material, such as gadolinium oxisulfide in a binder, or a tungstate, such as cadmium tungstate or calcium tungstate, in a binder. It can also be combined with a light absorbing material adapted to attenuate and adjust the light fluence delivered to the X-ray to charge converter, such as an organic light absorber dissolved in the binder used to bind the luminophor to the substrate. A light absorbing powder can also be incorporated into the binder of the luminophor for this purpose. Moreover, the red light emitted by the luminophor can be attenuated by a semitransparent layer deposited on the front side of the transparent substrate, which semitransparent layer may be a thin metallic film of titanium, gold, chromium or aluminum.